The invention relates to a process for the preparation of hydrocarbons in a catalytic reaction at elevated temperature and pressure employing a methanol-comprising feed.
The preparation of hydrocarbons from a mixture comprising hydrogen and carbon monoxide, by contacting this mixture at elevated temperature and pressure with a specific catalyst, is known in the literature as the Fischer-Tropsch hydrocarbon synthesis process. Catalysts used in the Fischer-Tropsch synthesis process usually contain one or more metals from the iron group together with one or more promotors, both deposited on a porous carrier material. The products that can be prepared using these catalysts usually have a very wide range of molecular weight distribution, a large variety in branched and unbranched paraffins, and often contain considerable amounts of olefins and oxygen-containing organic compounds. Usually, only a minor portion of these products comprise so-called middle distillates. Middle distillates relate to hydrocarbon mixtures of which the boiling point range corresponds substantially to that of kerosine and gas oil fractions obtained in a conventional atmospheric distillation of crude mineral oil. The boiling point range of middle distillates generally lies within the range of about 150.degree. to about 360.degree. C. Furthermore, these middle distillates are obtained at a relatively low yield while their pour point is also unsatisfactory. Accordingly, the Fischer-Tropsch hydrocarbon synthesis process is not a very attractive direct route for the production of middle distillates on a technical scale.
Recently, Fischer-Tropsch catalysts have been found, which comprise cobalt as the metal component originating from the iron group, and a promotor. Suitable promotors are zirconium, titanium, chromium, ruthenium, iron, magnesium, zinc, thorium and uranium, preferably are zirconium, titanium, chromium and ruthenium and mixtures thereof, most preferably zirconium and titanium. More in particular, these catalyst comprise
i) a porous carrier material selected from the group comprising silica, alumina, and mixtures thereof; PA1 ii) cobalt as a metal component deposited on the porous carrier material; and PA1 iii) a promotor selected from the group comprising zirconium, titanium, chromium, ruthenium, iron, magnesium, zinc, thorium and uranium. PA1 i) a porous carrier material selected from the group comprising silica, alumina, and mixtures thereof; PA1 ii) cobalt as a metal component deposited on the porous carrier material; and PA1 iii) a promotor selected from the group comprising zirconium, titanium, chromium, ruthenium, iron, magnesium, zinc, thorium and uranium.
These catalyst yield a product in which only very minor amounts of olefins and oxygen-containing organic compounds are present and which consists substantially completely of unbranched paraffins. A considerable portion of these paraffins boil above the boiling point range of the middle distillates. It has been found that these high boiling portions of this product may be converted at high yields into middle distillates by means of hydrocracking. As a feedstock for hydrocracking at least a portion of the product is chosen such that the initial boiling point lies above the final boiling point of the heaviest middle distillates desired as end product. Hydrocracking typically proceeding at very low hydrogen consumption, yields middle distillates having a considerably better pour point than those obtained by the direct conversion according to the Fischer-Tropsch hydrocarbon synthesis process.
In the preparation of hydrocarbons, mixtures of hydrogen and carbon monoxide in molar ratios varying between about 0.5-3, are used in the Fischer-Tropsch hydrocarbon synthesis process.
Recently, it has been found that a specific type of Fischer-Tropsch catalyst may be used for the conversion of a feed comprising methanol and optionally a mixture of hydrogen and carbon monoxide into hydrocarbons. These specific catalysts, disclosed in U.S. Pat. Nos. 4,568,663; 4,663,305; and 4,751,345 are titania supported cobalt catalysts, promoted with rhenium, rhenium and thorium, zirconium, hafnium, cerium or uranium. It appears that in such processes employing methanol as feed, the selectivity for products with 2 or more carbon atoms is less than 75%.
Surprisingly, it has been found that the above-indicated class of Fischer-Tropsch catalysts comprising silica, alumina, or mixtures thereof shows a high selectivity for heavy hydrocarbons and is therefore especially suitable for the preparation of very heavy hydrocarbons which heavy hydrocarbons are very suitable for the production of middle distillates in a two-step process.